The invention relates to a driver assistance system in a motor vehicle comprising a speed limiting function, including a so-called “escape function.”
Such a driver assistance system is known from DE 195 09 492 C2, for example. In this known driver assistance system comprising a speed limiting function, a maximally allowed top speed can generally be predefined. The speed limiting function automatically limits the vehicle speed to this maximum speed, even if the actuation of the gas pedal by the driver under normal circumstances would result in a vehicle speed that is higher than the set maximum speed. In this case, a corrective signal corresponding to the maximum speed is generated independently from the actual driving or gas pedal position, the signal being used to set the engine and the transmission. The specification of the maximum speed can be deactivated by a defined manual intervention on the part of the driver (“escape function”). If in this case the current gas pedal position significantly exceeds the last value of the corrective signal of the speed limiting function, the system attempts to increase the engine torque in the shortest possible time so much that the actual and the desired engine torque agree with each other again.
A certain “escape function” is also described in DE 10 2009 030 345 A1 in connection with a speed limiting function, by way of which the limitation to a predefined maximum speed can be overridden at the driver's input, whereby the maximum speed can be exceeded, at least temporarily, without a warning intervention (visually, acoustically or haptically).
Moreover, it is known that motor vehicles consume very little fuel when they “coast,” which is to say when they use their kinetic energy to roll as far as possible, without being decelerated by the drag torque of the engine (coasting mode). For example, a method for operating a drive train in a motor vehicle is known from DE 10 2009 057 551 A1, wherein the internal combustion engine is decoupled from the driving wheels while the vehicle is driving when predetermined operating conditions exist, which is to say a transition is made to a coasting operation or coasting mode.
Finally, vehicles with automatic transmissions are already available on the market, wherein the automatic transmission in an automatic operation of the transmission is switched to a coasting mode of the drive train when defined operating conditions exist by disconnecting the operative connection between the internal combustion engine and the driving wheels, such as by disengaging the torque converter lock-up clutch or a clutch in the transmission. If a manual upshift or downshift command is made by the driver during the coasting mode by way of operating elements provided for this purpose, the coasting mode is abandoned and a switch is made to normal operation of the drive train, which is to say the operative connection between the internal combustion engine and the driving wheels is restored.
It is the object of the invention to improve a driver assistance system comprising a speed limiting function with respect to increasing the acceptance thereof.
This object is achieved according to the invention by the subject matter of the independent claim(s). The dependent claims are advantageous refinements of the invention.
The invention is based on the following realization:
A number of criteria are decisive for modern drivers today so as to increase the acceptance of driver assistance systems. A driver assistance system should assist, but not intervene or warn unnecessarily. In addition, efficiency should be as high as possible, and fuel consumption should be as low as possible.
The method according to the invention takes these requirements into consideration, for example when returning from the above-mentioned “escape function,” which allows the basically predefined maximum speed to be temporarily exceeded.
Known driver assistance systems comprising a speed limiting function, such as a speed limit device (SLD), limit the vehicle speed to a value selected by the driver. For this purpose, typically no braking is carried out, but only the drive system or the driving torque is regulated.
The drive system is being increasingly optimized as part of energy-related and fuel-saving efficiency measures. For example, the above-mentioned coasting operation or a generally longer gear ratio is selected to reduce friction losses. This results in no or only low drag delays.
If a motor vehicle, equipped with (braking torque-reducing) efficiency measures and a speed limiting function, as a result of a certain condition temporarily drives faster than the maximum speed that is predefined by the driver, the vehicle is very slowly decelerated to the maximum speed due to the efficiency measures only if the particular condition is no longer present. The vehicle speed thus may remain above the maximum speed predefined by the driver for a long period. However, if this is the case, the customer's expectation for speed limitation is not met. In addition, according to general regulations (such as Euro NCAP requirements) a warning must be issued when the set maximum speed is exceeded too long so as to prompt the driver to manually brake. This lowers the benefit for the customer and is nonsensical from an energy perspective since the driver increases the friction losses again by actuating the brake system.
According to the invention, the braking torque is thus increased, in particular by influence on the driving torque or the drive drag behavior, when the maximum speed has been exceeded and must be reduced again as part of the limiting function. Increasing the braking torque shall also be understood to mean generating or allowing a braking torque when the starting point is a previously prevented braking torque.
In particular the following certain conditions may cause the maximum speed to be exceeded:                “escape function”: the activation of the above-mentioned escape function, in particular due to rapid or almost full depression of the gas pedal;        “reduction of the maximum speed”: the reduction of the maximum speed or the activation of the limiting function, with specification of a (new) maximum speed, which in each case is below the current actual vehicle speed;        “unintended acceleration”: the unrequested (unintentional) increase in the vehicle speed due to gradients, generally without actuation of the gas pedal, as long as the vehicle speed has not risen by more than a predefined comparatively low value above the maximum speed.        
If these conditions are no longer present, deceleration to the maximum speed must take place as quickly as possible. According to the invention, this is achieved by way of a function module for controlling the longitudinal dynamics, in particular for drag or braking torque generation of the drive system. For example, this function module may be included as a program module in an electronic control unit for controlling the internal combustion engine and/or electric motor and/or automatic transmission.
Deceleration to the maximum speed by increasing or generating a braking torque can be carried out either in a controlled or regulated manner; this means that at least one measure is carried out, and preferably multiple measures are carried out in stages, to increase or generate the braking torque for expedited deceleration. A particularly advantageous measure is to prohibit or deactivate the above-mentioned coasting operation. Moreover, in particular in the case of electric or hybrid vehicles, the activation of increased regenerative operation is particularly advantageous. Further measures can include deliberate downshifts in automatic transmissions or the deliberate activation of secondary consumers (such as generator or air conditioning compressor).
According to the invention, a function module is thus provided for the speed limiting function, in which certain conditions for exceeding the maximum speed and for decelerating again to the maximum speed are implemented. Moreover, the function module generates signals for carrying out measures to generate or increase the braking torque. For this purpose, a signal for carrying out a measure can be a binary signal (such as “coasting operation prohibited”/“coasting operation allowed”), which only increases the deceleration in the form of a control step. However, the signal for carrying out a measure can also be a required braking torque progression in the form of a regulating step so as to reach the maximum speed within a predefined time period. In the second case, the predefined time period is preferably shorter than a defined time window during which a warning intervention of the speed limiting function is suppressed during an exception where the maximum speed is allowed to be exceeded. In this way, both unpleasant warning interventions and the necessity for a manual braking intervention by way of friction are automatically prevented.
The drawings show one exemplary embodiment of the invention. The drawings show three examples for increasing a braking torque according to the invention after the certain conditions mentioned above by way of example have ended, which in exceptions can cause a maximum speed to be exceeded. In the drawings:
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.